Nickel-base superalloys are strengthened by the precipitation of the gamma-prime (γ′) phase. The gamma-prime phase is produced by heating the nickel-base superalloy to a temperature above the gamma-prime solvus to form a solid solution, quenching the solutionized alloy to low temperature, and then precipitation-heat-treating the solutionized-and-quenched alloy at an intermediate aging temperature. The result is a distribution of gamma-prime phase in a nickel-alloy matrix. A high volume fraction of gamma-prime phase is desired to provide high strength. However, a high volume of gamma-prime phase presents processing challenges because the material has low ductility at the processing temperatures.
Nickel-base superalloys used in creep-limited applications are desirably coarse grained. The coarse-grain microstructure is produced during the high-temperature solution treatment, because the grains rapidly coarsen at this temperature. The coarse-grain microstructure is more creep resistant than is a fine-grain microstructure. However, the coarse-grain microstructure is less ductile in intermediate temperature ranges than is the fine-grain microstructure, so that the coarse-grain microstructure may be subject to quench cracks during the quenching that follows the solution treatment. The desirable high-volume-fraction of gamma-prime phase makes the alloy even more prone to cracking due to the reduced ductility.
An additional problem is encountered when the nickel-base superalloy is utilized to make an article such as a disk (rotor) used in the turbine section of a gas-turbine engine. Such articles may have a widely varying section thickness, from relatively thin near the rim to relatively thick near the hub. When such an article is solution-treated-and-quenched, there is a significant variation in the cooling rate of the regions of different thicknesses, as well as between the center and the surface of the thick sections, leading to large residual strains and stresses within the article. These residual strains and stresses lead to distortion of the article during subsequent machining and service. While the residual strains and stresses may be relaxed somewhat by a stabilization heat treatment prior to the precipitation heat treatment, the stabilization heat treatment leads to a reduction in the strength properties of the article after aging.
There have been a number of processes developed to achieve good mechanical properties while alleviating the problems associated with the limited ductility of the coarse-grain microstructure. A fan cool from the solution treatment provides an intermediate cooling rate between a slow cooling rate (i.e., air cooling) that leads to reduced residual stress but also reduced strength, and a faster cooling rate (e.g., water quench, oil quench, one-step molten salt quench) that produces increased strength but also increased risk of cracking, distortion, and residual stresses. Other techniques include the use of differential cooling rates, where one portion of the article is cooled at a slower rate and another portion is cooled at a faster rate, as with jets of liquid or gas. While all of the prior techniques are operable to some extent, none has been found to be fully satisfactory in achieving a desirable compromise in mechanical properties with no cracking, low distortion, and low residual stresses.
There is accordingly a need for an improved approach to the solution treating, quenching, and precipitation heat treating of nickel-base superalloys, particularly those that have coarse grains and high volume fractions of gamma-prime phase, and are shaped as articles with thick sections and/or varying section thicknesses. The present invention fulfills this need, and further provides related advantages.